1. Field of the Invention
The present invention relates generally to the field of radiation-sensitive imaging devices, and more specifically to radiation-sensitive imaging devices that have improved immunity to x-ray noise.
2. Related Background Art
Increasingly, electronic imaging sensors have been replacing film-based sensors in commercial, industrial, and medical imaging applications. Examples of such electronic sensors include charge-coupled device (CCD) sensors and complementary metallic-oxide-semiconductor (CMOS) sensors, to name a few. CMOS sensors, in particular, have emerged as a preferred candidate due to advantages in manufacturing cost, integration of components, charge efficiency, and low power consumption. An excellent example of a detector system suitable for medical imaging applications that uses a CMOS active pixel sensor (APS) array is provided in U.S. Pat. No. 5,912,942 to David B. Schick et al., assigned to the assignee of the present patent application. The Schick '942 patent is incorporated herein by reference.
In the system of the Schick '942 patent, as in many medical imaging applications, radiation or energy from x-ray photons, commonly referred to as x-rays, is projected through a patient and must be registered or detected by a sensor. However, conventional sensors or imaging chips, which generally are fabricated from silicon, are considerably more sensitive to photon energy in the visible spectrum than to x-rays. Thus, a scintillator is disposed on the sensors to convert the energy from the x-rays to visible light. The scintillator is typically composed of gadolinium oxysulphide or cesium iodide, although other alternative materials may be used.
While it is desirous that the scintillator convert all of the x-rays to visible light, in practice only a percentage is actually converted, with the remaining x-rays passing through the scintillator and reaching the silicon. A typical gadolinium oxysulphide scintillator, for example, may have a so-called stopping efficiency of 20-30%, meaning that only 20-30% of the x-rays that impinge on the scintillator are converted to visible light, with a significant amount of the x-rays (some 70-80%) being transmitted through the scintillator into the surface of the sensor. Although the sensor may be designed to be primarily sensitive to visible light, it will nevertheless be secondarily sensitive to effects from the transmitted x-rays. As a result, the transmitted x-rays are registered as noise by the sensor, reducing the overall quality of the captured image.
To reduce such noise, conventional systems typically shield the sensor from transmitted (unconverted) x-rays. In the Schick '942 patent, for example, such shielding is achieved by interposing a fiber-optic plate (FOP) between the scintillator and the sensor. The FOP allows light to pass, but blocks (i.e., absorbs) a large amount of the unconverted x-rays. While generally good for its intended application, this shielding approach has the drawback of adding an undesirable thickness to the sensor, which compromises patient comfort. FOPs also cause some degree of light signal loss and light spreading, each of which reduces image quality. Also, as FOP sizes increase, they become extremely expensive and fragile. Accordingly, for many reasons, it is desirable to avoid using FOPs.
Recently, advanced x-ray-sensitive photoconductive materials, such as selenium, lead iodide (PbI2), and mercuric iodide (HgI2), for example, have allowed designers to produce sensors that can image x-rays directly, so that a conventional scintillator is not needed. When x-rays strike the surface of such a photoconductive material, electron-hole pairs are formed. These charges are driven by an electrical potential or bias potential, which causes charges of a selected polarity to be collected by a storage element, such as a capacitor. This alternative design has the benefit of limiting light-spreading, and may achieve preferred spatial resolution as compared to typical scintillators. However, although these photoconductive materials have superior stopping or conversion efficiency, their efficiency nevertheless is imperfect. And of course, because x-rays are directly imaged, an FOP generally is not used in these arrangements. Accordingly, with these constructions as well, an improved x-ray noise immunity is desirable.
Other structures for providing radiation shielding have been proposed. For example, U.S. Pat. No. 6,690,074 B1 to Dierickx is aimed at providing a radiation-resistant semiconductor device, and is particularly concerned with reducing the leakage current between the source and drain electrodes in a MOS-type structure, resulting from an overlap between the gate electrode and the field oxide. Towards this end, the Dierickx device uses a doped guard ring interrupted by an active area. This approach, however, while perhaps adequate for its intended purpose of preventing or reducing leakage currents, which typically occur near the surface of semiconductor devices, is completely ineffective at shielding from the deleterious effects of unconverted x-rays, which usually occur well below the surface.
There is a great need, therefore, for a semiconductor x-ray imaging chip that takes an entirely fresh approach, and provides improved x-ray noise immunity, and at the same time a superior image quality.